1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to a wireless communication system that determines access to wireless communications.
2. Description of the Prior Art
People and businesses are demanding higher bandwidths from their communication providers. Consequently, the communication providers are looking for ways to increase the bandwidth of their systems using broadband technologies. Broadband technologies are generally referred to as systems that deliver a bandwidth at or above 64 kbps. Broadband technologies can communicate over downstream channels and upstream channels. The customer receives data from another device or system over the downstream channels. The customer transmits data to another device or system over the upstream channels.
Broadband Wireline Systems
One example of a broadband technology is Digital Subscriber Line (DSL) service. DSL service can carry both voice signals and data signals at the same time in both directions. DSL service also can carry call information and customer data. DSL service is typically comprised of twisted-pair wires that connect a customer to a central office. The central office comprises a Digital Subscriber Line Access Multiplexer (DSLAM) that provides the DSL service to the customer. Unfortunately, the speed of DSL service is limited by the distance between the customer and the DSLAM. Customers located too far from the DSLAM may not be able to receive high-speed service. Also, there may not be enough customers within a particular area to make it economical to install a DSLAM. The quality of DSL service is also limited by the quality of the copper wire that connects the customer to the DSLAM. Furthermore, DSL service does not work over Digital Loop Carrier (DLC) lines.
Another broadband technology is cable modem service. The cable modem communicates with a device or system over a coaxial cable. The coaxial cable is typically the same coaxial cable used to receive cable television. The cable modem service can be one-way or two-way. In a two-way system, the coaxial cable carries both the upstream channels and the downstream channels. In a one-way system, the cable modem receives data on the downstream channels over the coaxial cable and transmits data on the upstream channels over a phone line. Unfortunately, the cable modem uses up valuable bandwidth on the phone line in the one-way system. Also, the upstream bandwidth is small over a phone line.
Broadband Wireless Systems
Another broadband technology is broadband wireless service. Customers that subscribe to broadband wireless service communicate with a head end. In a one-way wireless system, a transmitter antenna for the head end broadcasts wireless signals to the customer on the downstream channels. The transmitter antenna is a satellite antenna or a land-based antenna. The customer transmits data to the head end over another medium, such as a phone line or a cable modem, on the upstream channels. One example of a one-way wireless system is a Digital Satellite System (DSS) from DIRECTV.
A specific type of broadband wireless system communicates over Multichannel Multipoint Distribution Service (MMDS) frequencies and Multipoint Distribution Service (MDS) frequencies. The MMDS frequencies range from 2596 MHz to 2644 MHz. The MDS frequencies range from 2150 MHz to 2162 MHz. In a typical MMDS system, the bandwidth of the upstream channels is about 6 MHz. The upstream bandwidth is divided into subchannels. Each subchannel has a bandwidth of 200 kHz.
A head end manages the upstream and downstream channels with the customer. The head end also interfaces the customer with communication systems such as the Internet. The head end includes a base antenna comprised of a transmitter antenna and one or more receiver antennas. MMDS requires a line of sight between devices that are, communicating. Therefore, the antennas are placed on a high building or a mountain to establish lines of sight with the customers.
The transmitter antenna is omni-directional and broadcasts data from the head end to the customers on the downstream channels. In a two-way wireless system, the receiver antennas are positioned to receive MMDS signals transmitted from customers to the head end on the upstream channels. Each receiver antenna is positioned to receive MMDS signals from customers located within a certain area. The areas formed by the antennas are referred to as sectors. The sectors have designated frequency ranges or designated channels.
The head end is comprised of an upstream manager and a downstream manager that control transmissions on the upstream channels and the downstream channels, respectively. As stated above, the upstream channels and the downstream channels are divided into subchannels. One upstream subchannel is a contention channel reserved for signaling, while the remaining subchannels are bearer channels.
In the broadband wireless system, a wireless broadband router is located at a customer premises. The wireless broadband router communicates with the upstream manager and the downstream manager to exchange data. The upstream manager generally operates the channels and/or subchannels in four states: idle, contention, polling, and dedicated. In the idle state, the channels are idle. In the contention state, the upstream manager generates and transmits control signals over one or more subchannels.
For the polling and dedicated states, the upstream manager polls numerous wireless broadband routers to allocate use of the subchannels. Polling is a round robin process to determine access to a subchannel. The upstream manager maintains a queue of the active wireless broadband routers to determine which wireless broadband router is next to transmit over a subchannel for a period of time. The upstream manager keeps an inventory of open subchannels and waiting wireless broadband routers in the queue.
The upstream manager uses a credit to grant a wireless broadband router use of a subchannel for a limited period of time. A credit is a message that allows usage of a subchannel for a period of time or for the transfer of a maximum number of transmission units, such as bytes. One example of a credit is information, such as a subchannel or frequency range, a maximum allowed time to transfer data, and a maximum number of bytes the wireless broadband router is allowed to transfer.
There are two kinds of credits: polling and dedicated. Polling credits are credits related to polling of the wireless broadband routers. Polling credits are generally smaller than the dedicated credits. Once the wireless broadband router completes transfer of the packets, the wireless broadband router transmits a DONE message to the upstream manager via the upstream channels. The DONE messages include information such as the number of bytes sent and the number of packets left for the wireless broadband router to transfer. A DONE message is one example of a termination message. The termination message is any signaling, instruction, or message that indicates termination of access for a subchannel or completion of a transfer of information or data. Once the DONE message is received, the upstream manager then transmits another credit. If the DONE message shows that the wireless broadband router has more than three packets left to transfer and there are available subchannels, then the upstream manager issues a dedicated credit to the wireless broadband router.
One problem is the delay from transmitting a credit from the upstream manager to the wireless broadband router to the time the packet is transmitted from the wireless broadband router to the upstream manager. In this example, the delay is 25 ms. The upstream manager determines an open channel and an awaiting wireless broadband router. The upstream manager then transmits the credit with a destination Internet Protocol (IP) address of the awaiting wireless broadband router to the downstream manager. The delay for transmitting the credit from the upstream manager to the downstream manager is approximately 1 ms. The downstream manager performs framing of the credit into a Media Access control protocol, a forward error correction (FEC) function to disperse errors in a packet and interleaving, which in total delays the transmission of the credit by approximately 12 ms. The downstream manager encapsulates the credit in UDP/IP. The downstream manager then transmits the credit to the wireless broadband router. The wireless broadband router then receives the credit, performs FEC, interleaving and queuing on the upstream. The delay of the wireless broadband router is approximately 12 ms.
Thus, the total delay from the upstream manager transmitting the credit to receiving the packet is 25 ms. Once the DONE message is received, the upstream manager then transmits another credit.
In the best case scenario with a maximum sized packet, the utilization is only 70% with the 25 ms delay. The maximum transmission unit (MTU) is 1,500 bytes for this MMDS system, which is 12,000 bits/second. The maximum upstream transmission rate is 200,000 bits/sec. Thus, the time to transfer a maximum sized packet is 60 ms which is 12,000 bit/sec divided by 200,000 bits/sec. The total time to transfer a packet is 60 ms plus the 25 ms delay, which is 85 ms. Thus, the utilization is approximately 70% which is 60 ms divided by 85 ms.
In the worst case scenario with the minimum sized packet with a size of less than or equal to 120 bytes, the utilization is approximately 17%. The time to transfer 120 bytes is 5 ms. Thus, the total time to transfer a packet is 30 ms with the 25 ms delay. Thus, the utilization is approximately 17%, which is 5 ms divided by 30 ms.
In the average case scenario, 90% of all upstream packets are equal to or less than 120 bytes. Nearly 98% of upstream polling packets are equal to or less than 120 bytes, so the average case utilization is near 20%.
The polling ratio is a ratio of wireless broadband routers to channels. In this example, the polling ratio is 8:1 such as 80 wireless broadband routers to 10 channels. For small packets of 120 bytes, the inter-packet delay for a user at this packet size is 240 ms, which is 8 multiplied by 30 ms. Thus, the user can only transmit 4 packets per second. The best-case efficiency occurs when packet sizes are at their largest (1500 Bytes), and is 70%. The inter-packet delay in this case is 680 ms.
Another problem is the 25 ms delay reduces the utilization in many scenarios. Another problem is how to control traffic on the upstream bandwidth to maximize upstream utilization.